When Thomas Jefferson measured the temperature on July 4, 1776, the high was 76 degrees Fahrenheit in Philadelphia. Two hundred and fifty years later, millions of people across the eastern half of the U.S. were under extreme heat warnings as they celebrated that anniversary.
The heat dome that settled over the Midwest and Atlantic coast then gave way to multiple rounds of severe storms and flash flooding, according to the National Weather Service.
This extreme weather did not come as a surprise, and many cities across the country were prepared: Independence Day parades in Philadelphia and Washington, D.C., were canceled, while Boston did not open access to its annual fireworks event until 4 p.m.
Meanwhile, dozens of heat-related deaths were reported across the country and emergency rooms saw high numbers of people suffering from heat-related illnesses.
Extreme heat could be a mark of many Independence Days in the future, experts say. Climate change, caused primarily by burning fossil fuels, is making heat waves hotter and longer. The average number of heat waves in the U.S. has doubled since the 1980s.
"It's not an anomaly. It's a preview," said Michael Rawlins, associate director of the Climate System Research Center at the University of Massachusetts Amherst.
What do the numbers say?
On July 4, a large portion of the Eastern U.S., from New York to Georgia, had "extreme" rates of emergency department visits for heat-related illnesses, according to the Centers for Disease Control and Prevention's daily heat-related illness tracker.
At least 40 deaths have been reported by local officials in connection with the heat wave: 29 in New Jersey, 3 in New York, 4 in Philadelphia, and 4 in Illinois.
However, experts said heat-related deaths are hard to pinpoint. Heat-related deaths are undercounted and often heat exacerbates an underlying medical condition.
"Teasing out which of the deaths are due to extreme heat and which are due to other causes is not an exact science," said Steven Cohen, who is the director of the sustainability management program at Columbia University and a former policy analyst for the U.S. Environmental Protection Agency. There's no universal standard for determining the cause of death when it's associated with a climate-related disaster.
The heat also led to an increase in people seeking help from emergency responders.
In D.C., hundreds of people were treated who were at the National Mall celebration: DC Fire and EMS reported 96 patient contacts, George Washington University reported 289 patient contacts, and the U.S. Department of Health and Human Services reported 314 patient contacts. There were no specific figures available for heat-specific treatment, but the temperature hit 102 degrees in D.C. over the weekend.
At a Fourth of July event in Boston, 34 of the 108 people treated by emergency responders were cases associated with the heat, according to Caitlin McLaughlin, director of media and public relations at Boston Emergency Medical Services.
From Wednesday to Saturday, 84 of all 911 calls in Boston were associated with heat. Rawlins said the number shows a "public health system under real stress."
"These numbers are the human cost of what the climate data is telling us," Rawlins said.
And experts and officials said some of the numbers aren't even out of the ordinary for heat-related emergencies.
How will people adapt?
Overall, people need to understand that more extreme weather is going to happen — like it or not, Rawlins said.
"It's laudable that the systems are stepping up and parades are being canceled and cooling centers are opening up, but I wouldn't want anyone to get complacent in thinking that we're prepared for what the future may bring," he said, adding that the key to avoiding the worst consequences of climate change is to bring down greenhouse gas emissions.
Cohen said extreme weather events reflect the "new normal," and many cities are preparing. He sees changes like keeping public buildings with air conditioning open longer, improving local emergency response services, and real estate developers taking climate change into account.
Still, the planet will continue to get hotter, he said. "We're learning how to adapt to it. But there's a limit to how much we can adapt," Cohen said.
Both experts said people are also taking warnings more seriously. A 2025 poll from the University of Chicago found that "about 9 in 10 Americans who experienced extreme weather believe climate change is a contributing factor."
"There's more awareness of the health impacts of extreme weather," Cohen said. "People are paying attention to it because it's no longer something that happened to somebody else. It's something that happened to my cousin, my brother, my friend or myself."
In the future, Rawlins said this Fourth of July and other extreme weather events cannot be looked at in isolation. From June 1 through July 5, temperatures at weather stations across New England recorded their highest temperatures on record or just below their highest, he said.
"That's not a heat wave — that's a season," Rawlins said.
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